U.S. patent number 7,484,679 [Application Number 10/518,769] was granted by the patent office on 2009-02-03 for method of milling cerium compound by means of ball mill.
This patent grant is currently assigned to Nissan Chemical Industries, Ltd.. Invention is credited to Isao Ota, Noriyuki Takakuma, Kenji Tanimoto, Gen Yamada.
United States Patent |
7,484,679 |
Ota , et al. |
February 3, 2009 |
Method of milling cerium compound by means of ball mill
Abstract
A method of milling cerium compound by means of a ball mill
using a milling medium, characterized in that ratio H.sub.b/r of
radius r of a cylindrical ball mill container and depth H.sub.b of
the milling medium in the ball mill container disposed horizontally
ranges from 1.2 to 1.9, and the ball mill container is rotated at a
rotational speed which is 50% or less of critical rotational speed
N.sub.c=299/r.sup.1/2 of the ball mill container converted from the
radius r expressed in centimeter. The milling method can be carried
out in a wet or dry process, and the cerium compound is preferably
cerium oxide. The method can be also applied for producing a cerium
compound slurry.
Inventors: |
Ota; Isao (Nei-gun,
JP), Tanimoto; Kenji (Nei-gun, JP), Yamada;
Gen (Nei-gun, JP), Takakuma; Noriyuki (Nei-gun,
JP) |
Assignee: |
Nissan Chemical Industries,
Ltd. (Tokyo, JP)
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Family
ID: |
30112349 |
Appl.
No.: |
10/518,769 |
Filed: |
July 3, 2003 |
PCT
Filed: |
July 03, 2003 |
PCT No.: |
PCT/JP03/08475 |
371(c)(1),(2),(4) Date: |
December 21, 2004 |
PCT
Pub. No.: |
WO2004/004910 |
PCT
Pub. Date: |
January 15, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050253001 A1 |
Nov 17, 2005 |
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Foreign Application Priority Data
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Jul 4, 2002 [JP] |
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2002-195742 |
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Current U.S.
Class: |
241/21;
241/184 |
Current CPC
Class: |
B02C
17/00 (20130101); B02C 17/20 (20130101); C09K
3/1409 (20130101) |
Current International
Class: |
B02C
1/00 (20060101); B02C 11/08 (20060101); B02C
21/00 (20060101); B02C 23/00 (20060101) |
Field of
Search: |
;241/21,30,184 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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A 2000-109808 |
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Apr 2000 |
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JP |
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A 2002-186870 |
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Jul 2002 |
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JP |
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A 2002-212544 |
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Jul 2002 |
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JP |
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Other References
Patton, "Paint Flow and Pigment Dispersion," pp. 202-222, 1971.
cited by other .
Kano et al., Chemical Equipment, No. 9, pp. 50-54, 2001. cited by
other.
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Primary Examiner: Miller; Bena
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
The invention claimed is:
1. A method of milling cerium compound, comprising: providing the
cerium compound and a milling medium in a cylindrical ball mill
container of a ball mill, the milling medium having a H.sub.b/r
ratio between 1.2 and 1.9 where H.sub.b is the depth of the milling
medium in the ball mill container and r is the radius of the ball
mill container; and rotating the ball mill container at a
rotational speed 50% or less of a critical rotational speed of the
ball mill container, the critical rotation speed being defined as
N.sub.c=299/r.sup.1/2 where N.sub.c is expressed in rpm and r is
expressed in cm.
2. The method of milling cerium compound according to claim 1,
wherein the milling of the cerium compound is carried out as a wet
process.
3. The method of milling cerium compound according to claim 1,
wherein the cerium compound is cerium oxide.
4. The method of milling cerium compound according to claim 3,
wherein zirconium is used in an amount of 100 ppm to 10000 ppm
based on cerium (IV) oxide.
5. The method of milling cerium compound according to claim 1,
wherein the ball mill container is rotated at a rotational speed
that is from 10% to 50% of the critical rotational speed
N.sub.c.
6. The method of milling cerium compound according to claim 1,
wherein the radius r of the ball mill container is 5 to 50 cm.
7. The method of milling cerium compound according to claim 1,
wherein the milling medium is partially stabilized zirconia
ball.
8. The method of milling cerium compound according to claim 1,
wherein the milling medium has a diameter of 0.3 to 25 mm.
9. The method of milling cerium compound according to claim 1,
further comprising: providing a water-soluble alkaline silicate;
and adjusting the pH of the resulting slurry containing the cerium
compound to a pH from 8 to 13, wherein milling is carried out as a
wet process to obtain the cerium compound covered with amorphous
silica.
10. The method of milling cerium compound according to claim 9,
wherein the water-soluble alkaline silicate is lithium silicate,
sodium silicate, potassium silicate or quaternary ammonium
hydroxide silicate.
11. The method of milling cerium compound according to claim 1,
wherein the milling of the cerium compound is carried out as a dry
process.
Description
TECHNICAL FIELD
The present invention relates to a method appropriate for milling
cerium oxide particles by means of a ball mill.
BACKGROUND ART
Physical factors dominating milling effect of ball mill milling
equipments include dimension (radius r) and rotational speed rpm in
regard to a ball mill container. In regard to beads, amount of
filled beads (this is expressed in ratio H.sub.b/r of depth H.sub.b
of filled beads to radius r (cm) of the ball mill container, or the
ratio of the beads to the internal volume of the container), or
material, diameter and shape (spherical, cylindrical, etc.) of the
beads may be mentioned. Among these physical factors, it is known
that consumption power becomes maximum and the best milling
efficiency is obtained in case where the amount of filled beads
which is expressed in H.sub.b/r is 1.0 (corresponds to 50% based on
the internal volume of the ball mill container).
However, in case where the amount of filled beads is as little as
30% or less (H.sub.b/r of 0.6 or less), the balls start to slide
along the inner wall of the container to cause remarkable damages
to the inner wall. Therefore, in the actual production process, the
amount of beads is generally kept to one third to half of the total
volume of the ball mill container (H.sub.b/r of 0.66 to 1.0).
In the milling by a ball mill, the balls are gradually lifted
highly in the rotational direction with the movement of the mill,
and the ball is involved in a snowslide motion together with a
plenty of balls when the balls are lifted at the position where
there is no support below the balls. Consequently, the balls slide
and fall on the surface of the balls and fall below the mill while
they collide here and there (snowslide phenomenon).
When the rotational speed is increased, the balls come to fall like
a waterfall in the space filled with vapor, rather than the
snowslide phenomenon (waterfall phenomenon).
When the rotational speed is further increased, the mill comes to
be rotated while the balls are adhered to the inner wall of the
mill due to centrifugal force (adhesion phenomenon/adhesion
state).
It is clear that no dispersion is achieved in the adhesion state
(the balls do not move relatively with the mill). In addition, in
the state of the waterfall phenomenon, the balls and the inner wall
of the mill have many damages, and dispersion is insufficient.
Therefore, these phenomena are undesirable states, and the
dispersion of pigments is carried out very efficiently in only the
state of snowslide phenomenon which is regarded as an ideal
state.
In regard to the rotational speed of the container, it is stated
that the optimum rotational speed N.sub.0=(203-0.60r)/r.sup.1/2
wherein the unit of r is cm (RPM.sub.0=(37-3.3r)/r.sup.1/2 wherein
the unit of r is feet) at the point of which the snowslide
phenomenon occurs is an ideal state in the milling by a ball mill
(see, for example "Paint Flow and Pigment Dispersion" written by
Temple V. Patton, translation supervised by Kenji Ueki., Kyoritsu
Shuppan Co., Ltd., 1971, pp. 202-222). This publication states that
the above-mentioned equation expressing the optimum rotational
speed N.sub.0 at the point of which the snowslide phenomenon occurs
is obtained in case where the critical rotational speed
N.sub.c=60g.sup.1/2/2.pi.r.sup.1/2=299/r.sup.1/2, and is derived
from N.sub.0=(0.68-0.22r)N.sub.c (rpm.sub.0=(0.68-0.06r)rpm.sub.c
wherein the unit of r is feet). In addition, the publication states
that the actual production process is generally carried out in the
amount of filled beads and the rotational speed of the container as
mentioned above.
In addition, it is stated that the milling of aluminum hydroxide
powder is carried out in a ball mill made of stainless steel having
a diameter of 78 mm to 199 mm by means of steel beads having a
diameter of 10.2 mm (see, for example, "Chemical Equipment" written
by Sumiya Kano, Hiroshi Mio and Fumiyoshi Saito, 2001, No. 9, pp.
50-54). This publication reports the test results in which the
milling condition is as follows: bead-filling rate of 20 to 80% and
number of revolutions of 0.6 to 1.3 time the critical rotational
speed. As a result of it, it is stated that milling rate becomes
maximum when the bead-filling rate is 40 to 80% and number of
revolutions is 80% of the critical rotational number, and the
milling rate is increased with an increase in bead diameter, and
the milling rate is lowered when the bead-filling rate is beyond
60%.
In the meanwhile, cerium oxide particles are widely used as
polishing agent for substrates containing silica as main component,
and recently there is a strong demand for cerium oxide polishing
agent by which a polished face with a high quality can be obtained
without surface defects such as scratch. On the other hand, it is
also required strongly to maintain a high removal rate so as not to
decrease the productivity. Therefor unmilled large particles
causing scratch and over-milled fine particles causing a lowering
in removal rate must be reduced in the number in cerium oxide
particles to the utmost. That is, it is required a production
method by which the particle size distribution of cerium oxide
particles can be controlled in order to make it further sharp.
Cerium oxide particles have been finely divided by milling with
ball mill using a milling medium such as partially stabilized
zirconia oxide beads or alumina beads. However, as these beads are
very hard for cerium oxide and milling condition which is generally
achieved for milling it is too vigorous, particle size distribution
of cerium oxide fine particles becomes very broad.
The present invention resolves this problem and provides a milling
method for obtaining cerium oxide particles with a narrow particle
size distribution. The cerium oxide particles obtained according to
the present invention have a narrow particle size distribution.
Therefore, in case where it is used for polishing, it provides a
polished face with a high quality without lowering in removal rate,
and thus it makes possible to improve the production efficiency and
lower the cost.
DISCLOSURE OF INVENTION
The present invention includes the following aspects: as a first
aspect, a method of milling cerium compound by means of a ball mill
using a milling medium, characterized in that ratio H.sub.b/r of
radius r of a cylindrical ball mill container and depth H.sub.b of
the milling medium in the ball mill container disposed horizontally
ranges from 1.2 to 1.9, and the ball mill container is rotated at a
rotational speed which is 50% or less of critical rotational speed
N.sub.c=299/r.sup.1/2 of the ball mill container converted from the
radius r expressed in centimeter; as a second aspect, the method of
milling cerium compound as set forth in the first aspect, wherein
the milling of the cerium compound is carried out in wet process or
dry process; as a third aspect, the method of milling cerium
compound as set forth in the first aspect, wherein the cerium
compound is cerium oxide; as a fourth aspect, the method of milling
cerium compound as set forth in the first aspect, wherein the ball
mill container is rotated at a rotational speed which is 10% or
more of N.sub.c; as a fifth aspect, the method of milling cerium
compound as set forth in the first aspect, wherein the radius r of
the ball mill container is 5 to 50 cm; as a sixth aspect, the
method of milling cerium compound as set forth in the first aspect,
wherein the milling medium is partially stabilized zirconia ball;
as a seventh aspect, the method of milling cerium as set forth in
the first aspect, wherein the milling medium has a diameter of 0.3
to 25 mm; as an eighth aspect, the method of milling cerium
compound as set forth in the first aspect, wherein zirconium is
used in an amount of 100 ppm to 10000 ppm based on the cerium
compound in terms of cerium (IV) oxide; as a ninth aspect, the
method of milling cerium compound as set forth in the first aspect,
wherein a water-soluble alkaline silicate is added, pH of a slurry
containing the cerium compound is adjusted to 8 to 13, and then a
wet milling is carried out to obtain cerium compound covered with
amorphous silica; as a tenth aspect, the method of milling cerium
compound as set forth in the ninth aspect, wherein the
water-soluble alkaline silicate is lithium silicate, sodium
silicate, potassium silicate or quaternary ammonium hydroxide
silicate; and as an eleventh aspect, a method of producing a slurry
of cerium compound from an aqueous or organic solvent medium
containing cerium compound by means of a ball mill using a milling
medium, characterized in that ratio H.sub.b/r of radius r of a
cylindrical ball mill container and depth H.sub.b of the milling
medium in the ball mill container disposed horizontally ranges from
1.2 to 1.9, and the ball mill container is rotated at a rotational
speed which is 50% or less of critical rotational speed
N.sub.c=299/r.sup.1/2 of the ball mill container using the radius r
expressed in centimeter.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention relates to a method of milling cerium
compound by means of a ball mill using a milling medium,
characterized in that ratio H.sub.b/r of radius r of a cylindrical
ball mill container and depth H.sub.b of the milling medium in the
ball mill container disposed horizontally ranges from 1.2 to 1.9,
and the ball mill container is rotated at a rotational speed which
is 50% or less of critical rotational speed N.sub.c=299/r.sup.1/2
of the ball mill container converted from the radius r expressed in
centimeter.
The present invention may be carried out by milling powdery cerium
compound in dry process, or milling an aqueous or organic solvent
medium containing cerium compound in wet process.
That is, in the wet process, a slurry of cerium compound can be
produced according to a method of producing a slurry of cerium
compound from an aqueous or organic solvent medium containing
cerium compound by means of a ball mill using a milling medium,
wherein ratio H.sub.b/r of radius r of a cylindrical ball mill
container and depth H.sub.b of the milling medium in the ball mill
container disposed horizontally ranges from 1.2 to 1.9, and the
ball mill container is rotated at a rotational speed which is 50%
or less of critical rotational speed N.sub.c of the ball mill
container using the radius r expressed in centimeter.
In the present invention, cerium oxide is preferably used as cerium
compound. Cerium oxides to be placed in a ball mill container with
a polishing medium are cerium oxide particles with a particle
diameter of 0.1 .mu.m or more, preferably 0.1 to 100 .mu.m obtained
by calcining commercially available cerium carbonate in a shape of
hexagonal plate of several to ten-odd .mu.m at 400 to 1200.degree.
C. In addition, commercially available cerium oxide powders with a
mean particle diameter of 1 .mu.m or less or several .mu.m can be
also used.
In the meanwhile, cerium compounds are not limited to cerium
oxides, and water-insoluble cerium compound such as cerium
carbonate can be used.
As the potential of beads risen up by rotation of a ball mill
container becomes high with an increase in the radius of the ball
mill container, and the striking energy due to free fall thereof
becomes high, fine particles are apt to be obtained by
over-milling. When cerium compound, for example relatively soft
material such as cerium oxide is milled with a relatively hard
medium such as zirconia, the range of the above-mentioned radius r
is important. The ball mill container used in the present invention
preferably has the radius r ranging from 5 to 25 cm.
The amount of filled beads is set in such a way that ratio
H.sub.b/r of depth H.sub.b of the filled beads to radius r of the
ball mill container ranges from 1.2 to 1.9 (63 to 97% based on the
inner volume), which is a higher value than case where general
milling by means of a ball mill (for example, H.sub.b/r ranges from
0.63 to 1.0, 33 to 50% based on the inner volume) is carried out.
This makes it possible to mill in a condition which does not occur
a situation where snowslide phenomenon is repeated, wherein the
situation is regarded as an ideal condition in general
powder-milling.
When H.sub.b/r is set within the range from 1.2 to 1.9, material to
be milled (cerium compound in a dry-milling, an aqueous or organic
solvent slurry containing cerium compound in wet-milling) that is
placed with a milling medium in a ball mill, is placed in an amount
of the milling medium:the material to be milled of 1:0.5 to 1:1.2
in volume ratio. When the milling medium and the material to be
milled are placed in this ratio in a ball mill container, the
combined volume of both amounts to 65 to 99.5% based on the total
volume. The slurry to be milled is a slurry containing cerium
compound in an aqueous or organic solvent in a solid concentration
of 1 to 70% by weight.
In addition, the rotational speed of the ball mill container is 50%
or less of critical rotational speed, and 80% or less of the
optimum rotational speed N.sub.o=(203-0.60r)/r.sup.1/2 that occurs
a snowslide phenomenon by which dispersion is efficiently achieved.
Thus, the present invention excludes a condition occurring a
situation where snowslide phenomenon of beads is repeated, wherein
the situation is regarded as an ideal condition in general
powder-milling.
In the present invention, milling is achieved within the range of
10% to 50% of the critical rotational speed N.sub.c. The rotational
speed correspond to a rotational speed ranging from 20% to 80% of
the optimum rotational speed N.sub.o=(203-0.60r)/r.sup.1/2 at which
a snowslide phenomenon occurs. As mentioned above, the present
invention provides cerium compounds, particularly cerium oxide
particles having a narrow particle size distribution by selecting a
condition out of the milling condition which it is generally
regarded that milling is achieved in the highest effect. Further,
the wet milling can provide cerium oxide slurry.
As mentioned above, the milling of cerium compound in the present
invention utilizes milling media having small particle diameter and
is carried out in a low rotational speed of a ball mill, compared
with the optimum milling condition that is normally applied for
particles. This make it possible to narrow the particle size
distribution of cerium compound, particularly cerium oxide when it
is milled.
In a process using a sand grinder or an attritor in which beads are
compulsorily rotated with an arm or disc, the milling is carried
out in the condition of ratio H.sub.b/r of depth H.sub.b of the
filled beads to radius r of the ball mill container ranging from
1.2 to 1.9 (63 to 97% based on the inner volume). However, it is
difficult to avoid partial over-milling due to a compulsory
rotation of the milling media. Therefore, a large amount of fine
particles are produced, and it is difficult to obtain cerium oxide
particles with a sharp particle size distribution.
When the radius r of the ball mill container is over 50 cm, the
potential energy of beads risen up thereby becomes high, and the
striking energy thereof becomes high due to free fall. Therefore,
it is not preferable as over-milling occurs and the particle size
distribution of the resulting milled particles becomes broad. On
the other hand, when the radius r of the container is less than 5
cm, it is not preferable as milled amount per batch is too small
and the cost becomes very high. Consequently, the radius r of the
container preferably ranges from 5 cm to 50 cm, more preferably
from 10 cm to 40 cm.
When the ratio H.sub.b/r of depth H.sub.b of filled beads to radius
r of a cylindrical ball mill container is over 1.9 (97% based on
the inner volume), it is not economical as milling speed is
markedly lowered. The ratio H.sub.b/r of depth H.sub.b of filled
beads to radius r of a cylindrical ball mill container is
preferably 1.2 to 1.9 (the amount of filled beads is 63 to 97%
based on the inner volume), further it is more preferable that
H.sub.b/r is 1.2 to 1.7.
The material of beads is preferably partially stabilized zirconia,
alumina, mulite or silica, which is harder than cerium oxide. Among
them, partially stabilized zirconia that little beads are worn out
is most preferable.
The size of beads is preferably 0.3 to 25 mm.phi.. When the size of
beads is less than 0.3 mm.phi., its own weight of beads becomes too
light, and milling efficiency is markedly lowered. On the other
hand, when the size of beads is more than 25 mm.phi., the striking
energy of beads each other becomes high, and over-milling occurs
locally and fine particles are easily produced.
In case where milling is achieved by using partially stabilized
zirconia, it is not possible to avoid contamination of zirconium
element in a slurry of cerium compound after milling. When the
cerium compound is cerium (IV) oxide, zirconium element is
contaminated in an amount of 100 ppm to 10000 ppm based on cerium
(IV) oxide. But the element is present in the shape of zirconia
fine particle, the element itself can be utilized as polishing
agent.
The method of milling cerium compound according to the present
invention, particularly the method for producing cerium oxide
particles can be applied for wet milling or dry milling.
In the wet process, acid such as nitric acid, hydrochloric acid,
acetic acid or the like can be used as a water-soluble dispersant.
In the meantime, the wet milling for a long time causes a rise in
pH of an acid slurry, the pH approaches 5 that is the isoelectric
point of cerium (IV) oxide. Therefore, the slurry is liable to be
aggregated and lowered in grindability.
Thus, in the process of wet milling in the present invention, a
water-soluble alkaline dispersant containing silica is added to
cover cerium (IV) oxide particles with amorphous silica, and the
resulting slurry is adjusted to pH 8-13 that is higher than the
isoelectric point of cerium (IV) oxide. Thereby, cerium (IV) oxide
particles are charged negatively, and the slurry is always kept in
a dispersed state, and homogeneous wet milling can be carried out
for a long time. The water-soluble alkaline dispersant containing
silica includes a water-soluble alkaline silicate or silica sol,
such as lithium silicate, sodium silicate, potassium silicate,
quaternary ammonium hydroxide silicate, and can be added in an
amount of 0.001 to 1 in a weight ratio of
(SiO.sub.2)/(CeO.sub.2).
The material of the ball mill container according to the present
invention includes metal such as stainless steel, iron or the like,
ceramics such as alumina, mulite or the like, resin such as nylon,
polyethylene, polypropylene, engineering plastics or the like.
Containers made of resin are preferable taking contamination of
impurities on milling or hardness of material into account.
Cerium compounds obtained according to the present invention have
the particle diameter measured by centrifugal sedimentation method
ranging from 50 to 600 nm, and have a low rate of large particles
over 400 nm in the whole particles compared with those of the prior
milling method. Further, the cerium compounds have also a low rate
of fine particles less than 30 nm in the whole particles.
Consequently, the present invention can provide cerium compound
particles with a narrow particle size distribution.
In case where milling is carried out in the wet process, cerium
compound slurry that contains cerium compound with the
above-mentioned particle diameter and particle size distribution in
concentration of 10 to 60% by weight and that has pH of 3 to 11 is
obtained by milling cerium compound in concentration of 10 to 60%
by weight with an aqueous medium of pH 3-11 for 1 to 72 hours.
Particularly, it is useful for producing cerium oxide slurry from
an aqueous medium containing cerium oxide.
EXAMPLES
Hereinafter, the present invention is described based on examples.
The analytical methods adopted in the examples are as follows. (1)
pH Measurement A pH meter (manufactured by To a DKK Ltd., HM-30S)
was used for pH measurement. (2) Conductivity Measurement A
conductivity meter (manufactured by To a DKK Ltd., CM-30S) was used
for conductivity measurement. (3) Measurement of Particle Diameter
by Centrifugal Sedimentation Method A mean particle diameter of D50
was measured with a particle diameter measurement apparatus by
centrifugal sedimentation method (manufactured by Shimadzu
Corporation, CP-3), and it was regarded as a particle diameter
based on centrifugal sedimentation method. (4) Measurement of
Particle Diameter by Laser Diffraction Method A mean particle
diameter of D50 was measured with a particle diameter measurement
apparatus by laser diffraction method (manufactured by Malvern
Instruments Ltd., Mastersizer 2000), and it was regarded as a mean
particle diameter based on laser diffraction method. (5) Particle
Diameter Determined from Specific Surface Area Measured by Gas
Adsorption Method A sample obtained by drying a cerium oxide
aqueous slurry in a prescribed condition was subjected to a
specific surface area analyzer by nitrogen adsorption (manufactured
by Quantachrome Instruments, Monosorb Type MS-16) to measure the
specific surface area Sw (m.sup.2/g), and a particle diameter in
terms of spherical particle (particle diameter calculated through
BET method) was determined. (6) Measurement Method of Amount of
Small Particles In 50 ml centrifugal tube, 37 g of milled slurry
obtained by diluting to 17% by weight of solid content with pure
water was placed, the tube was centrifuged at 3000 rpm (G=1000) for
10 minutes, and then 22.5 g of supernatant was taken, and dried at
110.degree. C. to obtain powder. An amount of small particles was
determined by dividing the weight of the resulting powder by the
weight of solid content in the slurry prior to centrifugation. The
small pailicles were those less than 30 nm according to an
observation with transmission electron microscope. (7) Measurement
Method of BET Method-Based Particle Diameter of Large Particles In
100 ml glass sedimentation tube, 115 g of milled slurry obtained by
diluting to 15% by weight of solid content with pure water was
placed, and after one day, 2 ml of slurry was recovered from the
bottom. After drying the recovered slurry in a prescribed
condition, the specific surface area was measured similarly to the
procedure in (4) and the particle diameter based on BET method was
calculated, and it was regarded as particle diameter calculated
through BET method (BET method-based particle diameter) of large
particles. (8) Observation with Scanning Electron Microscope An
electron microscopic photograph of a sample to be observed was
taken with a scanning electron microscope (manufactured by JEOL
Ltd., FE-SEM S-4100), and the resulting photograph was observed.
(9) Measurement of Powder X-Ray Diffraction A X-ray diffraction
apparatus (manufactured by JEOL Ltd., JEOL JDX-8200T) was used for
measurement of powder X-ray diffraction. (10) Measurement of
Isoelectric Point of Cerium (IV) Oxide A slurry containing cerium
(IV) oxide in 1% by weight was prepared, and the isoelectric point
thereof was measured with Zetasizer HS 3000 (manufactured by
Malvern Instruments Ltd.) (11) Measurement of Removal Rate of
Thermal Oxidation Layer Film thickness of thermal oxidation layer
was measured with a film thickness analyzer NanoSpec (manufactured
Nanometrics Incorporated) before and after polishing, and removal
rate was determined.
Example 1
150 kg of commercially available cerium oxide having bar-shaped
particles of 0.2 to 3 .mu.m with an observation by a scanning
electron microscope, mean particle diameter based on laser
diffraction of 3.2 .mu.m and a specific surface area based on BET
method of 128 m.sup.2/g was calcined in 1 m.sup.3 gas calcination
furnace at 1100.degree. C. for 5 hours to obtain yellow-white
powder. The resulting powder was measured with X-ray diffraction
apparatus and main peaks were detected at diffraction angle
2.theta.=28.6.degree., 47.5.degree. and 56.4.degree. which were
consistent with characteristic peaks of cubic system crystalline
cerium oxide described in ASTM card 34-394. An observation with a
scanning electron microscope revealed that the calcined cerium
oxide powder was aggregated particles having a primary particle
diameter of 150 to 300 nm. In addition, the specific surface
thereof was 2.8 m.sup.2/g.
Partially stabilized zirconia beads of 1 mm.phi. were placed in an
amount of 59 kg in a polyethylene container having a dimension of
radius 15 cm.times.length 34 cm (in this point, H.sub.b/r=1.4,
amount of filled beads was 71%), and further 5.9 kg of the cerium
oxide powder obtained by calcination at 1100.degree. C., 11.8 kg of
pure water and 47 g of 10% nitric acid were placed therein. Then,
milling was carried out at a rotational speed of 30 rpm
corresponding to 39% of the critical rotational speed of this
container N.sub.C=77 rpm for 18 hours. This afforded a cerium (IV)
oxide aqueous slurry having solid content concentration of 33% by
weight, pH 5.9 and conductivity of 318 .mu.m/S. The powder obtained
by drying this slurry at 300.degree. C. had specific surface area
of 7.1 m.sup.2/g and BET method-based particle diameter of 117 nm.
In addition, the particle diameter thereof was 100 to 300 nm with
an observation by a scanning electron microscope, and the mean
particle diameter was 260 nm according to centrifugal sedimentation
method. Further, the proportion of small particles less than 30 nm
was 1.5% and the BET method-based particle diameter of large
particles was 140 nm. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 66% in the whole
particles. In addition, zirconium element was contained in 1300 ppm
based on cerium (IV) oxide.
Example 2
Zirconia beads of 1 mm.phi. were placed in an amount of 135 kg in a
ball mill container having polyethylene lining with a dimension of
radius 15 cm.times.length 73 cm (in this point, H.sub.b/r=1.4,
amount of filled beads was 70%), and further 13.5 kg of the cerium
oxide powder obtained by calcination at 1100.degree. C. in Example
1, 27.0 kg of pure water and 107 g of 10% nitric acid were placed
therein. Then, milling was carried out at a rotational speed of 35
rpm corresponding to 45% of the critical rotational speed of this
container N.sub.C=77 rpm for 16 hours. This afforded a cerium (IV)
oxide aqueous slurry having solid content concentration of 33% by
weight, pH 5.8 and conductivity of 350 .mu.m/S. The powder obtained
by drying this slurry at 300.degree. C. had specific surface area
of 7.3 m.sup.2/g and BET method-based particle diameter of 114 nm.
In addition, the particle diameter thereof was 100 to 300 nm with
an observation by a scanning electron microscope, and the mean
particle diameter was 280 nm according to centrifugal sedimentation
method. Further, the proportion of small particles less than 30 nm
was 1.3% and the BET method-based particle diameter of large
particles was 138 nm. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 63% in the whole
particles. In addition, zirconium element was contained in 1200 ppm
based on cerium (IV) oxide.
Example 3
Commercially available cerium carbonate powder having purity of
99.9% (mean particle diameter based on laser diffraction method of
38 .mu.m) was calcined in an amount of 1600 g in an electric
furnace at 350.degree. C. for 5 hours, and then the temperature of
the furnace was risen to 900.degree. C. followed by calcination at
900.degree. C. for 15 hours to obtain 800 g of yellow-white powder.
The resulting powder was measured with X-ray diffraction apparatus
and main peaks were detected at diffraction angle
2.theta.=28.6.degree., 47.5.degree. and 56.4.degree. which were
consistent with characteristic peaks of cubic system crystalline
cerium oxide described in ASTM card 34-394. An observation with a
scanning electron microscope revealed that the calcined cerium
oxide powder was aggregated particles having a primary particle
diameter of 100 to 200 nm. In addition, the specific surface
thereof was 4.6 m.sup.2/g. The isoelectric point of the cerium (IV)
oxide was pH=5.
To a mixed aqueous solution of 20 g of commercially available 25%
tetramethylammonium hydroxide and 165 g of pure water, 21 g of 95%
tetraethoxysilane was added with stirring by disper to obtain
tetramethylammonium hydroxide silicate aqueous solution being an
alkaline silicate having pH of 12.8, conductivity of 8110 .mu.m/S
and SiO.sub.2 concentration of 2.9% by weight.
Partially stabilized zirconia beads of 1 mm.phi. were placed in an
amount of 6 kg in a polyethylene container having a dimension of
radius 6.5 cm.times.length 23 cm (in this point, H.sub.b/r=1.2,
amount of filled beads was 60%), and further 578 g of the resulting
cerium oxide powder, 372 g of pure water and 206 g of
tetramethylammonium hydroxide silicate aqueous solution
corresponding to weight ratio (SiO.sub.2)/(CeO.sub.2) of 0.01 were
placed therein. Then, milling was carried out at a rotational speed
of 60 rpm corresponding to 50% of the critical rotational speed of
this container N.sub.C=120 rpm for 32 hours. After milling,
beads-separation was carried out with pure water to obtain a cerium
(IV) oxide aqueous slurry (A-1) having solid content concentration
of 20% by weight, pH 11.9 and conductivity of 1734 .mu.m/S. The
resulting cerium (IV) oxide had the isoelectric point of pH 3.8.
The powder obtained by drying this slurry at 300.degree. C. had
specific surface area of 15.2 m.sup.2/g and BET method-based
particle diameter of 55 nm. In addition, the particle diameter
thereof was 100 to 200 nm with an observation by a scanning
electron microscope, and the mean particle diameter was 113 nm
according to laser diffraction method. The proportion (%) that the
particle diameter of the resulting particles fell within the mean
particle diameter according to laser diffraction method .+-.30% was
59% in the whole particles. The proportion of small particles less
than 30 nm was 7.9% and the BET method-based particle diameter of
large particles was 70 nm. In addition, zirconium element was
contained in 2760 ppm based on cerium (IV) oxide.
Comparative Example 1
Zirconia beads of 1 mm.phi. were placed in an amount of 25.1 kg in
a polyethylene container having a dimension of radius 15
cm.times.length 34 cm (in this point, H.sub.b/r=0.66, amount of
filled beads was 30%), and further 2.5 kg of the cerium oxide
powder obtained in Example 1, 5.0 kg of pure water and 20 g of 10%
nitric acid were placed therein. Then, milling was carried out at a
rotational speed of 30 rpm corresponding to 39% of the critical
rotational speed of this container N.sub.C=77 rpm for 12 hours.
This afforded a cerium (IV) oxide aqueous slurry having solid
content concentration of 33% by weight, pH 5.9 and conductivity of
318 .mu.m/S. The powder obtained by drying this slurry at
300.degree. C. had specific surface area of 7.4 m.sup.2/g and BET
method-based particle diameter of 113 nm. In addition, the particle
diameter thereof was 30 to 300 nm with an observation by a scanning
electron microscope, and the mean particle diameter was 290 nm
according to centrifugal sedimentation method. Further, the
proportion of small particles less than 30 nm was 2.5% and the BET
method-based particle diameter of large particles was 163 nm. The
proportion (%) that the particle diameter of the resulting
particles fell within the mean particle diameter according to laser
diffraction method .+-.30% was 41% in the whole particles.
Comparative Example 2
Zirconia beads of 1 mm.phi. were placed in an amount of 169 kg in a
nylon container having a dimension of radius 37 cm.times.length 73
cm (in this point, H.sub.b/r=0.42, amount of filled beads was 15%),
and further 16.7 kg of the cerium oxide powder obtained in Example
1, 33.8 kg of pure water and 134 g of 10% nitric acid were placed
therein. Then, milling was carried out at a rotational speed of 12
rpm corresponding to 25% of the critical rotational speed of this
container N.sub.C=49 rpm for 13 hours. This afforded a cerium (IV)
oxide aqueous slurry having solid content concentration of 33% by
weight, pH 5.5 and conductivity of 248 .mu.m/S. The powder obtained
by drying this slurry at 300.degree. C. had specific surface area
of 7.2 m.sup.2/g and BET method-based particle diameter of 116 nm.
In addition, the particle diameter thereof was 25 to 300 nm with an
observation by a scanning electron microscope, and the mean
particle diameter was 290 nm according to centrifugal sedimentation
method. Further, the proportion of small particles less than 30 nm
was 3.0% and the BET method-based particle diameter of large
particles was 168 nm. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 39% in the whole
particles.
Comparative Example 3
Zirconia beads of 1 mm.phi. were placed in an amount of 135 kg in a
nylon container having a dimension of radius 15 cm .times.length 73
cm (in this point, H.sub.b/r=1.4, amount of filled beads was 70%),
and further 13.5 kg of the cerium oxide powder obtained in Example
1, 27.0 kg of pure water and 107 g of 10% nitric acid were placed
therein. Then, milling was carried out at a rotational speed of 45
rpm corresponding to 58% of the critical rotational speed of this
container N.sub.C=77 rpm for 12 hours. This afforded a cerium (IV)
oxide aqueous slurry having solid content concentration of 33% by
weight, pH 6.3 and conductivity of 92 .mu.m/S. The powder obtained
by drying this slurry at 300.degree. C. had specific surface area
of 7.2 m.sup.2/g and BET method-based particle diameter of 116 nm.
In addition, the particle diameter thereof was 30 to 300 nm with an
observation by a scanning electron microscope, and the mean
particle diameter was 340 nm according to centrifugal sedimentation
method. Further, the proportion of small particles less than 30 nm
was 2.3% and the BET method-based particle diameter of large
particles was 160 nm. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 45% in the whole
particles.
Comparative Example 4
Partially stabilized zirconia beads of 1 mm.phi. were placed in an
amount of 6 kg in a polyethylene container having a dimension of
radius 6.5 cm.times.length 23 cm (in this point, H.sub.b/r=1.2,
amount of filled beads was 60%), and further 578 g of the cerium
oxide powder obtained by calcining in a similar condition as that
of Example 3, 372 g of pure water and 206 g of tetramethylammonium
hydroxide silicate aqueous solution prepared in Example 4
corresponding to weight ratio (SiO.sub.2)/(CeO.sub.2) of 0.01 were
placed therein. Then, milling was carried out at a rotational speed
of 90 rpm corresponding to 75% of the critical rotational speed of
this container N.sub.C=120 rpm for 16 hours. After milling, bead
separation was carried out with pure water to obtain a cerium (IV)
oxide aqueous slurry (B-1) having solid content concentration of
20% by weight, pH 11.3 and conductivity of 1725 .mu.m/S. The powder
obtained by drying this slurry at 300.degree. C. had specific
surface area of 15.0 m.sup.2/g and BET method-based particle
diameter of 56 nm. In addition, the particle diameter thereof was
30 to 300 nm with an observation by a scanning electron microscope,
and the mean particle diameter was 113 nm according to laser
diffraction method. The proportion (%) that the particle diameter
of the resulting particles fell within the mean particle diameter
according to laser diffraction method .+-.30% was 43% in the whole
particles. The proportion of small particles less than 30 nm was
8.8% and the BET method-based particle diameter of large particles
was 74 nm. In addition, zirconium element was contained in 2900 ppm
based on cerium (IV) oxide.
TABLE-US-00001 TABLE 1 Item (I) (II) (III) (IV) (V) (VI) (VII)
Example 1 15 1.4 30 117 1.5 140 66 Example 2 15 1.4 35 114 1.3 138
63 Example 3 6.5 1.2 60 55 7.9 70 59 Comparative Example 1 15 0.66
30 113 2.5 163 41 Comparative Example 2 37 0.42 12 116 3.0 168 39
Comparative Example 3 15 1.4 45 116 2.3 160 45 Comparative Example
4 6.5 1.2 90 56 8.8 74 43
In table 1, item (I) is radius (cm) of ball mill container, item
(II) is H.sub.b/r ratio, item (III) is rotational speed (rpm), item
(IV) is BET method-based particle diameter (nm) of cerium oxide
aqueous slurry, item (V) is proportion (%) of small particles less
than 30 nm in the whole particles, item (VI) is BET method-based
particle diameter of large particles, and item (VII) is proportion
(%) in the whole particles that the particle diameter of the
resulting particles fell within the mean particle diameter
.+-.30%.
To aqueous sols (A-1, B-1) obtained in Example 3 and Comparative
Example 4, ammonium polyacrylate was added in a concentration of
100% by weight based on cerium (IV) oxide, and then polishing
compositions (a-1, b-1) were prepared by diluting the resulting
mixture with pure water in a manner that the solid content of
cerium (IV) oxide would be 1% by weight.
Polishing by means of the prepared polishing compositions was
carried out as follows: Polishing machine: a machine manufactured
by Techno Rise Corporation; Polishing pad: a polishing pad IC-1000
made of closed formed polyurethane resin (manufactured by Rodel
Nitta Company); Material to be polished: thermal oxidation layer on
4-inch silicon wafer; Number of revolutions: 60 rpm; Polishing
pressure: 500 g/cm.sup.2; and Polishing time: 2 minutes.
The assessment of polished faces shown in Table 2 were carried out
with an optical microscope, in which the case where fine defects
were observed was indicated by symbol (.DELTA.) and the case where
no defect was observed was indicated by symbol
(.circleincircle.).
TABLE-US-00002 TABLE 2 Removal rate (.ANG./min) Polished Face a-1
800 .circleincircle. b-1 750 .DELTA.
It can be pointed out that cerium oxide aqueous slurries in
Examples 1 to 2 and Comparative Examples 1 to 3 shown in Table 1
have BET method-based particle diameter ranging form 113 to 117 nm
which is approximately equal one another. However, the comparison
between Examples 1 to 2 and Comparative Examples 1 to 2 reveals the
followings. Comparative Examples 1 and 2 having a law ratio
H.sub.b/r of depth H.sub.b of filled beads to radius r of the ball
mill container (a law filling rate of beads) contain small
particles less than 30 nm in a high rate in the whole particles,
and the large particles thereof have a large BET method-based
particle diameter and therefore they contain a large amount of
large particles. Thus, it is understood that Comparative Examples 1
and 2 have a broader particle size distribution than Examples 1 and
2.
In addition, Comparative Example 3 in which a rotational speed of
the ball mill container was adjusted to a high value has a high
rate of small particles less than 30 nm in the whole particles and
a large BET method-based particle diameter of large particles.
Therefore, it is understood that Comparative Example 3 has a broad
particle size distribution.
Example 3 containing tetramethylammonium hydroxide silicate aqueous
solution as dispersant has a higher rate (%) in the whole particles
of particles falling within mean particle diameter according to
laser diffraction method .+-.30% than Comparative Example 4 in
which a rotational speed of the ball mill container was adjusted to
a high value. Therefore, it is understood that Example 3 has a
narrow particle size distribution. In addition, it is understood
that Example 3 contains has a narrow particle size distribution
also from the facts that it contains small particles less than 30
nm in a low rate in the whole particles and has a small BET
method-based particle diameter of large particles. Further, as
shown in Table 2, it is understood from the comparison in polishing
characteristics between Example 3 and Comparative Example 4 that
Example 3 has a higher removal rate and provides a better quality
of polished face.
Although a relationship between removal rate and smoothness of
polished face is generally in an opposite manner, particles in
cerium compound slurry obtained according to the present invention
contain small particles less than 30 nm in a law rate of 10% or
less in the whole particles and particles falling within mean
particle diameter according to laser diffraction method .+-.30% in
a high rate (%) of 50% or more in the whole particles, thereby the
present invention makes it possible to provide a high removal rate
and a good smoothness.
INDUSTRIAL APPLICABILITY
The present invention relates to a method of milling cerium (IV)
oxide particles. The milling method of the present invention
provides cerium oxide particles that contain a small amount of fine
particles and large particles and have a sharp particle size
distribution. Therefore, in case where the cerium oxide particle
obtained according to the present invention are used as polishing
agent for substrates containing silica as a main component, such as
rock crystal, quartz glass for photomask, glass hard disk or
oxidation layer of semiconductor devices, polished faces with a
high accuracy and smoothness can be efficiently obtained with a
high polishing speed and little scratch.
Further, in case where an aqueous sol containing cerium (IV) oxide
particle covered with amorphous silica is used for particularly
polishing substrates containing silica as a main component, such as
rock crystal, quartz glass for photomask, glass hard disk or
oxidation layer of semiconductor devices, it is hard to produce
residues and a good polished surface can be obtained.
* * * * *